iCloud backup

Have you gone to Settings/Your Name/iCloud, signed in with the same Apple ID, and turned Photos syncing on? Have you allowed time for the sync to complete? If you go to iCloud.com/Photos using a computer browser, are the photos there?

Try Settings/Your Name/iCloud and slide off syncing. Wait a few minutes and then slide syncing on. Allow time for syncing to occur.

Last resort - have you tried signing out of iCloud and then sign back in?

If you weren't syncing photos with iCloud, have you restored your backup?

Restoring from a backup erases all data currently on your device. Any data newer than when the backup was made will be erased. A restore is an all or nothing restore.

Backup - Restore your device from an iCloud or iTunes backup

Have a look at these support articles for more information

Get help with missing photos - Apple Support

Get help with iCloud Photos - Apple Support

Do you have an iTunes backup? Is this on iPhone?

If you have an iCloud backup, you should be able to restore it simply through the iCloud settings menu in your settings app.